Add colcon_core.logging.get_effective_console_level function

[cottsay]

When colcon routes log messages to log files at a different level from the console, it makes it a little more convoluted to determine what log level is actually set.

When we're utilizing non-colcon libraries that also use python's logging module (e.x. ros_buildfarm, rosdistro, etc), we'll typically want to "synchronize" colcon's configured log level with the other library. This function can be used to determine what level colcon's console logging is set to.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 83.24%. Comparing base (db84706) to head (0513457).

Files	Patch %	Lines
colcon_core/logging.py	80.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #650 +/- ## ======================================= Coverage 83.23% 83.24% ======================================= Files 66 66 Lines 3842 3849 +7 Branches 758 760 +2 ======================================= + Hits 3198 3204 +6 Misses 556 556 - Partials 88 89 +1 ```

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